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# According to a certain estimate, the depth N(t), in centimeters, of

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Director
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According to a certain estimate, the depth N(t), in centimeters, of  [#permalink]

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23 Jan 2012, 22:37
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According to a certain estimate, the depth N(t), in centimeters, of the water in a certain tank at t hours past 2:00 in the morning is given by N(t)= -20(t - 5)^2 + 500 for 0 ≤ t ≤ 10. According to this estimate, at what time in the morning does the depth of the water in the tank reach its maximum?

a) 5:30
b) 7:00
c) 7:30
d) 8:00
e) 9:00

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Re: According to a certain estimate, the depth N(t), in centimeters, of  [#permalink]

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24 Jan 2012, 02:45
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Baten80 wrote:
According to a certain estimate, the depth N(t), in centimeters, of the water in a certain tank at t hours past 2:00 in the morning is given by N(t)= -20(t-5)^²+500 for 0≤t≤10. According to this estimate, at what time in the morning does the depth of the water in the tank reach its maximum?

a) 5:30
b) 7:00
c) 7:30
d) 8:00
e) 9:00

Don't get bogged down by the dirty N(t) expression. Just think of it this way:

N(t) is a combination of two terms: a positive term (500) and a negative term ($$-20(t-5)^2$$).
To maximize N(t), I need to make the positive term as large as possible (It is a constant here so I cannot do much with it) and the absolute value of the negative term as small as possible. The smallest absolute value is 0. Can I make it 0? Yes, if I make t = 5, the negative term becomes 0 and N(t) is maximized. My answer must be 2:00 + 5 hrs i.e. 7:00.

Most of the maximum minimum questions on GMAT will require you to only think logically. The calculations involved will be minimum.
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Try ORION Free! ##### General Discussion Intern Joined: 27 Dec 2011 Posts: 6 Re: According to a certain estimate, the depth N(t), in centimeters, of [#permalink] ### Show Tags 23 Jan 2012, 22:44 1 the expression is N(t)= -20(t-5)^²+500 (of course valid after 2:00 in the morning) "the depth would be maximum" means the value of the above expression should be maximum or the value of square term (which has a negative 20 attached to it) should be minimum i.e. zero the square part is zero at t=5 so the time at which the depth is maximum is 2:00 + 5 hrs= 7:00 (B) Director Status: No dream is too large, no dreamer is too small Joined: 14 Jul 2010 Posts: 549 Re: According to a certain estimate, the depth N(t), in centimeters, of [#permalink] ### Show Tags 23 Jan 2012, 22:52 puneetkr wrote: the expression is N(t)= -20(t-5)^²+500 (of course valid after 2:00 in the morning) "the depth would be maximum" means the value of the above expression should be maximum or the value of square term (which has a negative 20 attached to it) should be minimum i.e. zero the square part is zero at t=5 so the time at which the depth is maximum is 2:00 + 5 hrs= 7:00 (B) if -20(t-5)^²=0 then t = 5 But Why is the 500 of the equation is not considered? Show your complete calculation. _________________ Collections:- PSof OG solved by GC members: http://gmatclub.com/forum/collection-ps-with-solution-from-gmatclub-110005.html DS of OG solved by GC members: http://gmatclub.com/forum/collection-ds-with-solution-from-gmatclub-110004.html 100 GMAT PREP Quantitative collection http://gmatclub.com/forum/gmat-prep-problem-collections-114358.html Collections of work/rate problems with solutions http://gmatclub.com/forum/collections-of-work-rate-problem-with-solutions-118919.html Mixture problems in a file with best solutions: http://gmatclub.com/forum/mixture-problems-with-best-and-easy-solutions-all-together-124644.html Intern Joined: 27 Dec 2011 Posts: 6 Re: According to a certain estimate, the depth N(t), in centimeters, of [#permalink] ### Show Tags 23 Jan 2012, 23:01 this is the complete calculation N(t)= -20(t-5)^²+500 now for any value of t we put in; we get some negative value of -20(t-5)^² (say -x) so our expression is now N(t)=500-x this expression would have maximum value only when x is minimum we know the minimum value for a square term is "zero" and (x has a square term) and that comes when t=5 i.e. when we put t=5 here we get N(t) = 500-0 = 500 Math Expert Joined: 02 Sep 2009 Posts: 47920 Re: According to a certain estimate, the depth N(t), in centimeters, of [#permalink] ### Show Tags 24 Jan 2012, 01:35 6 1 Baten80 wrote: puneetkr wrote: the expression is N(t)= -20(t-5)^²+500 (of course valid after 2:00 in the morning) "the depth would be maximum" means the value of the above expression should be maximum or the value of square term (which has a negative 20 attached to it) should be minimum i.e. zero the square part is zero at t=5 so the time at which the depth is maximum is 2:00 + 5 hrs= 7:00 (B) if -20(t-5)^²=0 then t = 5 But Why is the 500 of the equation is not considered? Show your complete calculation. According to a certain estimate, the depth N(t), in centimeters, of the water in a certain tank at t hours past 2:00 in the morning is given by N(t)= -20(t-5)^²+500 for 0≤t≤10. According to this estimate, at what time in the morning does the depth of the water in the tank reach its maximum? A. 5:30 B. 7:00 C. 7:30 D. 8:00 E. 9:00 Consider this: $$-20(t-5)^2\leq{0}$$ hence $$500-20(t-5)^2$$ reaches its maximum when $$-20(t-5)^2=0$$, thus when $$t=5$$. 2:00AM+5 hours=7:00AM. Answer: B. Hope it helps. _________________ Director Status: No dream is too large, no dreamer is too small Joined: 14 Jul 2010 Posts: 549 Re: According to a certain estimate, the depth N(t), in centimeters, of [#permalink] ### Show Tags 28 Jan 2012, 03:48 1 Intern Joined: 07 Dec 2012 Posts: 10 Re: According to a certain estimate, the depth N(t), in centimeters, of [#permalink] ### Show Tags 27 Nov 2015, 22:58 Can someone please explain why A is not the answer? Because if I plug in t = 3.5 (which is 5.30 (3.5 after the 2:00) , it gives me the maximum value of -20(t-5)^2 expression, which will be positive. Hence, the height will be at its max of 545. Hence, shouldn't the answer be A? Please explain the reasoning. Thank you Math Expert Joined: 02 Sep 2009 Posts: 47920 Re: According to a certain estimate, the depth N(t), in centimeters, of [#permalink] ### Show Tags 28 Nov 2015, 08:33 herein wrote: Can someone please explain why A is not the answer? Because if I plug in t = 3.5 (which is 5.30 (3.5 after the 2:00) , it gives me the maximum value of -20(t-5)^2 expression, which will be positive. Hence, the height will be at its max of 545. Hence, shouldn't the answer be A? Please explain the reasoning. Thank you Please read the solutions above. Also, if t = 3.5, then 500 - 20(3.5 - 5)^2 = 455, not 545. _________________ Board of Directors Status: QA & VA Forum Moderator Joined: 11 Jun 2011 Posts: 3757 Location: India GPA: 3.5 WE: Business Development (Commercial Banking) Re: According to a certain estimate, the depth N(t), in centimeters, of [#permalink] ### Show Tags 28 Nov 2015, 09:25 1 Baten80 wrote: According to a certain estimate, the depth N(t), in centimeters, of the water in a certain tank at t hours past 2:00 in the morning is given by N(t)= -20(t - 5)^2 + 500 for 0 ≤ t ≤ 10. According to this estimate, at what time in the morning does the depth of the water in the tank reach its maximum?0 N(t)= $$-20(t - 5)^2$$ + 500 ; The red highlighted portion of the equation is the most critical part, further t = 2am + time past 2am Now why $$-20(t - 5)^2$$ is important ; Any value of t less than 5 will result in a negative value of $$-20(t - 5)^2$$ Lets check : Time : 3am , t = 1 { t = 2am + time past 2am } $$-20(t - 5)^2$$ $$-20(1 - 5)^2$$ $$-20(-4)^2$$ $$-20(16)$$ $$-320$$ When put in the final equation : $$-20(t - 5)^2$$ the result will be -320 + 500 => 180 Check a few more any value of t less than 5 will result in -ve value of $$-20(t - 5)^2$$ and will ultimately lead to a depth of water less than 500. Our target is getting t = 5 ; since at t = 5 $$-20(t - 5)^2$$ will be 0 and the final equation $$-20(t - 5)^2$$ + 500 will be maximum ; ie 500 We already know t = 2am + time past 2am So, 7 = 2am + 5 hours. Answer is definitely (B) _________________ Thanks and Regards Abhishek.... PLEASE FOLLOW THE RULES FOR POSTING IN QA AND VA FORUM AND USE SEARCH FUNCTION BEFORE POSTING NEW QUESTIONS How to use Search Function in GMAT Club | Rules for Posting in QA forum | Writing Mathematical Formulas |Rules for Posting in VA forum | Request Expert's Reply ( VA Forum Only ) EMPOWERgmat Instructor Status: GMAT Assassin/Co-Founder Affiliations: EMPOWERgmat Joined: 19 Dec 2014 Posts: 12177 Location: United States (CA) GMAT 1: 800 Q51 V49 GRE 1: Q170 V170 Re: According to a certain estimate, the depth N(t), in centimeters, of [#permalink] ### Show Tags 28 Nov 2015, 16:07 1 1 Hi All, These specific types of "limit" questions are relatively rare on Test Day, although you'll likely be tested on the concept at least once. Whenever you're asked to minimize or maximize a value, you should look to do something with the other "pieces" of the equation (usually involving maximizing or minimizing those pieces). In the given equation, notice how you have two "parts": the -20(something) and a +500. Here, to MAXIMIZE the value of N(t), we have to minimize the "impact" that the -20(something) has on the +500. By making that first part equal 0, we'll be left with 0 + 500. Mathematically, we have to make whatever is inside the parentheses equal 0.... (T-5) = 0 T = 5 Since T represents the number of hours past 2:00am, we know that at 7:00am, the water will reach 500cm (the maximum value). Final Answer: GMAT assassins aren't born, they're made, Rich _________________ 760+: Learn What GMAT Assassins Do to Score at the Highest Levels Contact Rich at: Rich.C@empowergmat.com # Rich Cohen Co-Founder & GMAT Assassin Special Offer: Save$75 + GMAT Club Tests Free
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Re: According to a certain estimate, the depth N(t), in centimeters, of  [#permalink]

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14 Apr 2018, 17:49
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A useful trick to solve Maximum of any equation,with a single vraiable:

Just differentiate the equation with the variable and equate it to zero to find the solution. That solution is the value at which the euqation will have optimum value.

N(t)= -20(t-5)^²+500

d/dt N(t)= -40(t-5)=0

-40t=-200 => t=5

So, 7 = 2am + 5 hours.
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Re: According to a certain estimate, the depth N(t), in centimeters, of  [#permalink]

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04 Jun 2018, 03:38
valardohaeris wrote:
A useful trick to solve Maximum of any equation,with a single vraiable:

Just differentiate the equation with the variable and equate it to zero to find the solution. That solution is the value at which the euqation will have optimum value.

N(t)= -20(t-5)^²+500

d/dt N(t)= -40(t-5)=0

-40t=-200 => t=5

So, 7 = 2am + 5 hours.

Hi,
I used that method too.

I would like to know if there are always questions like this on every exam.

BR
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Re: According to a certain estimate, the depth N(t), in centimeters, of  [#permalink]

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05 Jun 2018, 10:32
Baten80 wrote:
According to a certain estimate, the depth N(t), in centimeters, of the water in a certain tank at t hours past 2:00 in the morning is given by N(t)= -20(t - 5)^2 + 500 for 0 ≤ t ≤ 10. According to this estimate, at what time in the morning does the depth of the water in the tank reach its maximum?

a) 5:30
b) 7:00
c) 7:30
d) 8:00
e) 9:00

Since -20(t - 5)^2, will be a nonpositive number, its maximum value is 0 when t = 5, and the maximum value of the function will then be:

N(5) = -20(5 - 5)^2 + 500 = 500

Thus, the maximum depth is at 2am + 5 hours = 7am.

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Re: According to a certain estimate, the depth N(t), in centimeters, of &nbs [#permalink] 05 Jun 2018, 10:32
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